1. Field of the Invention
The invention relates to a ventilator to be connected to the airways of a human or an animal patient for supplying and emitting respiratory gas to and from the airways, of the type having an inspiration section through which the respiratory gas is supplied to the airways, the inspiration section having at least one control valve for the gas flow and a safety valve, which is biased by a force load in the closing direction and a spring device in the opening direction, the safety valve being located between the control valve and the airways.
2. Description of the Prior Art
A known ventilator (7200 Microprocessor Ventilator, Puritan Bennett Corp., USA) is provided with an inspiration section, which is supplied with air and any additional gas from a compressed-air system. The respiratory gas passes through a system of control valves in order to obtain a desired pressure. To prevent an undesirable high pressure in the lungs of a patient, the inspiration section is provided with a safety valve between the control valves and the patient. The safety valve is described in the ventilator service manual, pages 80-82 and accompanying drawing and includes a valve that is biased in the opening direction by a spring and in the closing direction by an overpressure caused by air from the compressed-air system, adjusted to a pressure corresponding to the pressure of the spring plus the maximum allowed pressure for the patient. The amount of closing pressure is dependent on the pressure of the compressed-air system, because the valve that admits the pressure to the safety valve only has two fixed positions: open and closed. The safety valve thus opens at an overpressure that exceeds the maximum allowed pressure for the patient. Furthermore, the safety valve automatically opens in the event of a power failure and upon a shortage of pressure from the compressed-air system, which makes it possible for the patient to get air through the open safety valve.
As the closing force of the safety valve is dependent on a valve, there are risks for operational disturbances, should this valve become inoperative. The closing pressure could, for example, disappear if the valve ceases to function, and the safety valve would then open completely in spite of a correct respiratory gas pressure with respect to the patient.